Formerly possessed by the African-American community it slipped into the hands of wiggers at some point during the nineties. Sometime after this, it briefly appeared as though the word would be adapted into the white lexicon. However, it never gained the mainstream usage of other black words/phrases. Today, it is still used by the aforementioned wiggers (though usage of "truth" or some bastardization are common as well). A recent trend in usage can also be found among "above the fray" whites using irony (because you wouldn't expect them to say it, get it?). Which brings us to, "That staff meeting/ABBA concert was tight." "Word!"
Note: In the event of white usage, the word must be said with stereotypical "black" intonations. Also note that whites very rarely use "word" in the form of a question.
"Word?" ("You speak the truth?")
"Word." ("I speak the truth.")
"Word" has no single meaning, but is used to convey a casual sense of affirmation, acknowledgement, agreement, or to indicate that something has impressed you favorably.
Its usage among young blacks has been parodied ad nauseam among clueless suburban whites.